Tailoring Enhanced Elasticity of Crystalline Coordination Polymers

The approach for enhancing the elasticity of crystals with suboptimal elastic performances through a rational design was presented. A hydrogen-bonding link was identified as a critical feature in the structure of the parent material, the Cd(II) coordination polymer [CdI2(I-pz)2]n (I-pz = iodopyrazine), to determine the mechanical output and was modified via cocrystallization. Small organic coformers resembling the initial organic ligand but with readily available hydrogens were selected to improve the identified link, and the extent of strengthening the critical link was in an excellent correlation with the delivered enhancement of elastic flexibility materials.


Co-crystal synthesis
The targeted co-crystals were synthesized using conditions modified from reported procedures for the preparation of Cd(II) coordination polymers. 2 A reaction mixture of 1 (1 eq.) and organic co-former (1 eq.) was placed in a 10 mL stainless steel jar with 40 µL of ethanol and 2 stainless steel balls (7 mm in diameter) and ground for 60 minutes at 25 Hz frequency. Grinding was carried out in the Retsch MM200 ball mill. Once the solvent had evaporated the ground mixture was analyzed by powder X-ray diffraction (PXRD). The formation of a co-crystalline substance was confirmed by a comparison of experimental PXRD traces of the resulting grinding products with those of the starting substances.  Figure S4).

Growing crystals
The resulting grinding product was dissolved in methanol (1:A) or acetonitrile (1:B) at room temperature, and the solution was placed in a test tube, closed by parafilm (with a few holes to allow slow evaporation), and left undisturbed at ambient conditions to obtain single crystals suitable for testing mechanical adaptability to external mechanical force. Within a week, colorless (1:A) and yellowish (1:B) needle-like crystals of the required quality were harvested.
The phase purity of the final products was examined by analysis of the powder X-ray diffraction (PXRD) patterns; the PXRD of the bulk crystals was performed and compared with the calculated powder pattern of the co-crystal, 1:A and 1:B.

Powder X-ray crystallography
Polycrystalline samples were finely ground and placed on a silicon plate for powder X-ray diffraction (PXRD) experiments which were performed on a Malvern Panalytical Aeris powder diffractometer in the Bragg-Brentano geometry with PIXcel 1D detector under an applied voltage of 40 kV and current of 15.0 mA. The radiation used was CuKα (l = 1,5406 Å), and all patterns were collected at room temperature, from 4° to 60° (2) and with a step size of 0.02°. The PXRD data were collected for the resulting grinding products, 1:A ( Figure S1) and 1:B ( Figure S2), as well as for the bulk re-crystallization samples, 1:A ( Figure S3) and 1:B ( Figure S4).

Single crystal X-ray crystallography
Suitable crystals for single-crystal X-ray experiments were isolated from the mother liquor and mounted in a random orientation on a glass fiber. Data collections were carried out on an XtaLAB Synergy-S Dualflex diffractometer with PhotonJet (Mo) microfocus X-ray source and HyPix-6000HE hybrid photon counting (HPC) X-ray area detector and applying the CrysAlisPro Software system 3 at 295(2) K. Data reduction, including absorption correction, was done by CrysAlisPro program. The structures were solved by SHELXT program. 4 The coordinates and the anisotropic thermal parameters for all non-hydrogen atoms were refined by full-matrix least-squares methods based on F 2 using the SHELXL program. The hydrogen atoms were generated geometrically using the riding model with the isotropic factor set at 1.2Ueq.
Graphical work has been performed by Mercury 2021.3.0 software (version 4.3.1). 5 The thermal ellipsoids were drawn at the 50% probability level. General and crystal data with the summary of intensity data collection and structure refinement for compounds 1:A and 1:B are given in Table  S1.

Thermal analysis
Thermogravimetric analysis was performed using a simultaneous TGA-DTA analyzer Mettler-Toledo TGA/DSC 3+. The powder samples (1: A, 1:B) were placed in alumina pans (70 μL) and heated in flowing nitrogen (50 mL min −1 ) from room temperature up to 600 °C at a rate of 10 °C min −1 . Data collection and analysis were performed using the program package STARe Software v.16.30. 7 The mass losses that occurred in the first steps of both compounds (

Crystal bending experiments
For conducting experiments on crystal bending, needle-like crystals were isolated from the mother liquor and placed on a glass slide with a few drops of paratone oil added. A modified three-point bending procedure was employed using a pair of metal tweezers; the crystal was anchored from one side in two points and thus supported, while the mechanical force was applied with a metal needle from the opposite side (perpendicular to the elongation of the crystal) in a controlled manner by motorized moving the metal needle in regular increments (d = 30 m) and at constant velocity (v = 100 m/s). The parameters for calculating the bending strain value (): the distance between two points (L) where the crystal was anchored and the maximal displacement (hmax) of the crystal at the moment of maximal curvature, were measured just before the crystal fracture (Figs. S9, S10, S12, S13). The thickness of the crystal (t) was measured at the straight crystals' parts, while the radius of the curvature (R) was calculated from the geometric construction that approximated the crystal curvature by a circle (Scheme S1, Equation S1 , Equation S2). The elastic flexible response was then quantified using the Euler-Bernoulli equation 8 (considering pure bending without shear component, Equation S3). (%) = 2 • 100 Equation S3 Scheme 1. A three-point bending experiment in schematic representation: the distance between two supporters (blue double arrow, L); maximal displacement (red double arrow, hmax). The maximal curvature of the bent crystal was approximated by a circle of radius R (black line, R).